In certain machines such as, astronomical apparatuses and more particularly telescopes, it is important to be able to measure in an extremely precise manner the angular position of a component which is displaceable in rotation in relation to a fixed component.
This is so, for example, in the case of a telescope mounting which is displaceable in rotation within a supporting structure for the same and more particularly where it is desired to measure the azimuth angle or the elevation angle of this mounting.
In fact, these measurements must be made in an extremely precise manner with a very high resolution.
In the prior art, a certain number of devices are already known which permit the measurement of the angular position of a component which is movable in relation to a fixed component.
Thus, for example, it is possible to make use of an incremental optical encoder in the extension of the movable component, one of the parts of this encoder being fixed and being connected to the fixed component, while the other part is mounted so as to be displaceable in rotation within the fixed part and is connected to the movable component.
The output of this encoder is connected to means for determining the angular position of the movable component in relation to the fixed component on the basis of the relative angular position of the two encoder parts.
Any coupling means may be employed to connect the movable part of the encoder and the movable component of which it is necessary to measure the angular position in relation to the fixed component.
The structure of these coupling means must comply with a certain number of requirements in order not to disturb the measurement, while accepting certain defects of coaxiality between the components, these defects being due, for example, to the imperfect nature of the supporting bearings for the movable component. This leads to the construction of coupling means which are extremely complex and costly and which are relatively difficult to implement and maintain.
Furthermore, it is not always easy to have direct access to the end of the movable component in order to connect this end to the movable part of the encoder.
In order to solve the latter problem, use has been made of other devices comprising an encoded strip fixed, for example, to the periphery of the movable component. This strip is associated with any reading means which are disposed facing this strip, for example on the fixed component, and the output of which is connected to the means for determining the angular position of the movable component in relation to the fixed component.
However, the precision of these devices is dependent on the size of the assembly.
Furthermore, these devices are extremely sensitive to the clearances for mounting of the movable component within the fixed component, to thermal expansions or to the mechanical stresses to which these two components are subjected, so that in order to obtain an acceptable precision it is necessary to multiply the reading means at the periphery of the strip and to undertake comparisons and an in situ calibration of these devices in relation to a conventional standardization incremental optical encoder.
The validation of these calibrations requires a fidelity of the observed phenomena; in certain applications, this is impossible to obtain, so that the desired precision cannot always be obtained with these devices.